1. Field of the Invention
The present invention relates to a piezoelectric element that demonstrates a piezoelectric effect or a converse piezoelectric effect, and to a product featuring same, and more particularly to an improved manufacturing method that allows heretofore unattainable increases in the thickness of piezoelectric thin films to be achieved, and to a thin-film structure obtained by this manufacturing method.
2. Description of the Related Art
Piezoelectric ceramics have electromechanical conversion action. Piezoelectric elements are constructed by sandwiching piezoelectric thin films, which consist of such piezoelectric ceramic crystals, between electrodes.
In conventional practice, piezoelectric thin film precursors are commonly formed using a sol-gel technique or the like, and the piezoelectric thin films are ultimately crystallized by heat treatment at high temperatures.
Such heat treatments cause columnar crystals to grow from the bottom electrode inside the precursor films, yielding high-quality crystalline piezoelectric thin films having prescribed piezoelectric characteristics.
When crystallized by conventional high-temperature heat treatments, however, piezoelectric thin films are apt to crack, and are therefore disadvantageous in that they cannot be formed in a prescribed thickness (for example, 1 xcexcm or greater).
Such cracking is attributed to the fact that when crystals gradually grow in a piezoelectric precursor film, stress is applied in the direction parallel to the direction of film surface, splitting the crystals in the film.
In view of these drawbacks, a first object of the present invention is to provide a method for manufacturing a piezoelectric element and an ink-jet recording head in which the piezoelectric thin films can be made thicker.
A second object of the present invention is to provide a piezoelectric element formed from a thick film and provided with excellent piezoelectric characteristics, and to provide an ink-jet recording head and printer equipped with this piezoelectric element.
The method for manufacturing a piezoelectric element aimed at attaining the first object is a method for manufacturing a piezoelectric element having electromechanical conversion action, wherein this method for manufacturing a piezoelectric element comprises crystallizing a piezoelectric thin film by a process in which a piezoelectric precursor film containing the metal elements of a piezoelectric ceramic is heat-treated in a diffusion furnace.
The heat treatment in the diffusion furnace should preferably be performed by passing a reaction gas, and oxygen in particular, in a direction roughly parallel to the substrate surface provided with the piezoelectric precursor film. This is because aligning the substrate parallel to the oxygen stream allows a large number of such substrates to be lined up inside the diffusion furnace, and the oxygen to be fed uniformly in relation to the substrates. The cost can be reduced because a large number of substrates can be heat-treated together.
The heat treatment in the aforementioned diffusion furnace should preferably be performed every time piezoelectric precursors are stacked 0.1 to 0.5 xcexcm. Heat-treating the piezoelectric precursors every time they are stacked 0.2 to 0.4 xcexcm is particularly preferred.
A single cycle of heat treatment in the diffusion furnace may, for example, be performed for 10 to 60 minutes at 500 to 800xc2x0 C.
The piezoelectric element aimed at attaining the second object is provided with one or more layers in which fine crystal grains are present in a prescribed density (for example, 1.4xc3x97105 cmxe2x88x921) parallel to the film surface. In particular, the fine crystal grains should preferably have an annular shape.
In addition, the piezoelectric thin film for the piezoelectric element of the present invention should preferably have columnar crystal grains and fine crystal grains; each columnar crystal grain should extend in the direction of film thickness, and a plurality thereof should be aligned in the direction of film surface, yielding a layer of columnar crystal grains; and a plurality of such columnar crystal grain layers should be stacked in the direction of film thickness, yielding layers in which a plurality of fine crystal grains are aligned parallel to the film surface between the aforementioned layers. Another preferred option is for the fine crystal grains to be formed along the grain boundaries between the layers of columnar crystal grains and to be disposed along the borders between the layers. The presence of such fine crystal grains is believed to act as a buffer between the crystals and to have a crack-preventing effect.
In the piezoelectric thin film, the ratio of the mean grain size of the columnar crystal grains to the thickness of the piezoelectric thin film is set to 0.1 or lower. In other words, the piezoelectric thin film is fashioned to a thickness ten or more times greater than the grain size of the columnar crystal grains.
The mean grain size of the fine crystal grains is set to 50 nm or less. Each of the layers containing fine crystal grains has a thickness of 50 nm or less, as measured from the grain boundaries of the crystals. Layers containing fine crystal grains are present along the borders formed by a heat treatment in a diffusion furnace or the like.
Lattice-defect layers containing dislocations and lattice strain are present along the layers containing fine crystal grains. The thickness of such layers is 20 nm or less.
The present invention resides in an ink-jet recording head in which the piezoelectric element of the present invention is used as a piezoelectric actuator.
The present invention also resides in a printer in which the ink-jet recording head of the present invention is used as a printing means.